This invention generally relates to communication systems which require coherent detection of pulse or pulse compressed signals and in particular to an adaptive doppler tracker for use in such systems that provides simultaneous detection of interference signals.
Communications systems such as the proposed USAF SEEK TALK system employ coherent doppler trackers for base line systems. The use of this type of circuit is particularly advantageous in that: (a) there is no doppler tracking loss during data detection even at highest doppler rates; (b) coherent detection is optimum which is particularly important since the modem must operate at low signal-to noise ratios; (c) once a coherent reference is established it is relatively easy to generate a coherent signal for the array and (d) a frequency indicator is provided which is useful for identifying the same signal in adjacent range bins.
Coherent doppler trackers can be implemented in either analog or digital form. Breakthroughs in analog/digital converter technology, large scale integration of digital circuitry and the operational stability of digital processing make digital implementation attractive.
A standard technique for coherent data detection in systems of this type is the Costas receiver. This receiver uses a synchronous detecter and a phase locked oscillator to provide a coherent reference for the received signal. This type of receiver, however, operates with rectangular coordinate X,Y pulse components and consequently requires complex signal processing. It does not measure coherence between an estimated phase and the input signal phase and it utilizes a constant bandwidth through all modes of operation.
The foregoing difficiencies are largely overcome by the adaptive doppler tracker disclosed in copending U.S. patent application Ser. No. 329,558 of Allen W. Roeder entitled Sample Data Phase Locked Loop For Adaptive Doppler Trackers filed on even date herewith and incorporated herein by reference. However, in addition to the limitations noted, an additional problem is frequently encountered. That is the problem of processing interfering or simultaneously arriving signals. The present invention is directed toward solving this problem by adding on to the adaptive doppler tracker disclosed in the above referenced patent application, additional circuitry that detects the interference condition and provides for demodulation of both interfering signals with a single side band (SSB) processor.